1. Field of the Invention
The present invention relates to a fiber interferometer and more particularly to the control of the optical phase of a fiber interferometer of an interferometric fiber optic sensor by an optically induced change in the refractive index of the fiber in one arm of the fiber interferometer and utilizing the dependency of this optically induced change to provide passive-all optical phase shift interrogation.
2. Description of Related Art
Interferometric fiber optic sensors are known to be highly sensitive devices for measuring weak time varying fields such as acoustic pressure, vibration, magnetic fields, etc. because of their geometric versatility in the sensing element, wide dynamic range and extremely high sensitivity. The advantages which result from using the interferometric fiber optic sensors are attributed to the fact that the measurement technique employed measures the optical phase in the interferometer.
To achieve remote passive optical interrogation of fiber interferometers, a number of "demodulation" techniques based on phase generated carrier (PGC) concepts exist. These demodulation techniques conventionally use an unbalanced interferometer and laser frequency modulation to induce high frequency phase shift "carriers" in the interference output of the interferometric fiber optic sensor and the high frequency phase shift carriers are encoded by the lower frequency "signal" phase-shift information of interest. A passive (sine/cosine) homodyne technique, which requires some form of phase compensation to eliminate pre-detection signal fading caused by random differential temperature or mechanical fluctuations encountered by interferometers operating in normal environments, and a synthetic heterodyne technique, which has an infinite range because no pre-detection signal fading occurs but requires additional bulky and expensive optical components to produce the carrier signal in the interferometer output, are both possible in conventional interferometric sensors.
However, a problem of laser frequency jitter (induced phase noise) occurs when using these conventional forms of demodulation. Due to the required use of an unbalanced interferometer for operation in the PGC demodulation scheme, the interferometer is also inherently susceptible to laser frequency jitter (laser phase noise). As a result, the phase sensitivity obtained in the conventional sensor system is limited.